DOES THE UVI ON POLAR DETECT COSMIC SNOWBALLS

If 20 to 40 ton cosmic snowballs pelt Earth as claimed by Frank and Si gwarth [1997a], dark pixels will be produced in the 130.4 nm images of dayglow obtained by the Ultraviolet Imager (UVI) on the Polar spacecr aft. Examination of the UVI images has revealed that dayglow images ar e indeed spotted with single and multiple dark pixels. But is a snowba ll the only explanation for these dark pixels? To learn more about the dark pixels, we have examined the calibration images obtained from th e same camera just before the instrument was launched. We find that da rk pixels similar to those in dayglow images also exist in calibration images. This strongly indicates that the source of the dark pixels is instrumental. For further verification, a statistical analysis found the dark pixels from dayglow and calibration images have nearly identi cally shaped occurrence patterns. We have also looked for evidence of spacecraft ''wobble'' which demonstrates that the source of a bright o r dark feature in the images is external to the camera, but found none for dark pixels. Finally, we studied the bright streaks that frequent ly appear in UVI images, sometimes comet-like in appearance. These tra ils are ionization tracks produced by cosmic rays or other penetrating energetic particles interacting with our camera. We conclude that the source of the dark pixels in dayglow images is internal to the camera system and there is no scientific evidence from UVI that snowballs pe lt Earth.